1. Field of the Invention
The present invention relates to a current control circuit with limiter, and more particularly, to a current control circuit with limiter for generating a current proportional to an input voltage when the input voltage is smaller than a predetermined voltage and generating a constant current when the input voltage is equal to or higher than the predetermined voltage. The present invention also relates to a temperature control circuit using the current control circuit with limiter and a brightness control circuit using the current control circuit with limiter.
2. Description of the Related Art
FIG. 15 shows a conventional example of a current control circuit with limiter. A control input voltage Vi is applied to the non-inverting input terminal of an operational amplifier (hereinafter referred to as “OP-Amp”) 1 which operates as a voltage follower. The output of Op-Amp 1 is connected to the non-inverting input of an Op-Amp 6. The output of the Op-Amp 6 is connected to the base input of an NPN transistor 5. The emitter output of the NPN transistor 5 is connected to the inverting input of Op-Amp 6 and is supplied to a load resistance R1.
The entire circuit which is composed of Op-Amp 6, the NPN transistor 5, and the load resistance R1 operates as a voltage follower. The inverting input of Op-Amp 6, that is, the emitter output of the NPN transistor 5 becomes equal to the non-inverting input of Op-Amp 6.
A divided voltage produced by resistor R3 and resistor R4 is applied to the base of a PNP transistor 7. The divided voltage is called a reference voltage (hereinafter referred to as “Vr”). The output terminal of the emitter follower composed of the PNP transistor 7 is connected with the inverting input terminal of Op-Amp 1 and the non-inverting input terminal of Op-Amp 6. The inverting input terminal of Op-Amp 1 is connected with the output terminal thereof. A resistor R2 is a bias resistor for the output of Op-Amp 1 and also acts as a load resistor of the emitter follower composed of PNP transistor 7.
In the current control circuit with limiter, when Vi is lower than Vr+Vbe (Vbe is the base-emitter voltage of transistor 7, which is the voltage from the emitter to the base), the transistor 7 goes into its Off-state. At this time, Op-Amp 1 operates as a normal voltage follower. Therefore, the emitter voltage of transistor 5 at an output stage follows Vi, so an output current I=Vi/R1 proportional to Vi can be produced as the emitter current.
On the other hand, when Vi becomes equal to or higher than Vr+Vbe, transistor 7 goes into its On-state, so that the non-inverting input of Op-Amp 6 is clamped to Vr+Vbe, which is the emitter voltage of transistor 7. At this time, the emitter voltage of transistor 5 at the output stage is also clamped to Vr+Vbe. Therefore, the output current I becomes a constant value Ilimit=(Vr+Vbe)/R1, so that this circuit operates as a limiter.
FIG. 16 shows another conventional example of a current control circuit with limiter. In FIG. 16, portions identical to those in FIG. 15 are shown with the same reference numbers. In the circuit shown in FIG. 16, a voltage follower circuit composed of an Op-Amp 3 is provided instead of the emitter follower composed of the PNP transistor 7 as shown in FIG. 15. The output terminal of Op-Amp 1 is connected with a diode 2 and the output terminal of the Op-Amp 3 is connected with a diode 4.
When the circuit shown in FIG. 16 is used, the influence of Vbe of the transistor 7 which is caused in the circuit shown in FIG. 15 can be removed. This is because, the non-inverting input voltage of the Op-Amp 3 becomes Vr and the output of the Op-Amp 3 becomes Vr−Vf (Vf is the forward voltage of the diode 4), so that the voltage follower circuit composed of Op-Amp 3 is stabilized. At this time, the output of the voltage follower is Vr, so it is not influenced by diode 4.
A voltage limiter circuit which includes two Zener diodes oppositely connected in series to be able to control a limiter voltage of both polarities with high precision is disclosed in Japanese Utility Model Laid-open No. Hei 5-006929.
The above-mentioned circuit shown in FIG. 15 has a disadvantage that a limiter voltage is varied due to a variation in Vbe of transistor 7, which is caused by temperature.
In order to overcome this disadvantage, as shown in FIG. 16, Op-Amp 3 is used instead of the transistors, so that the influence of Vbe of the transistor due to temperature dependence can be removed. However, in the circuit shown in FIG. 16, the number of Op-Amps becomes three, with the result that a circuit scale increases. Therefore, there is a problem with respect to reductions in size and cost.
In the circuit shown in FIG. 16, diode 2 and diode 4 are subjected to wired OR connection. When the circuit serves as a limiter, it is necessary that diode 4 be in an On-state. Therefore, Vr can be set to only a limiter voltage corresponding to the forward voltage Vf of diode 4 as a minimum output voltage. This is because the output of Op-Amp 3 is equal to or higher than 0 V, so it is necessary that the inverting input, that is, Vr be equal to or higher than Vf. Thus, when the limiter voltage is lower than Vf, the circuit shown in FIG. 16 cannot be used.
Because the Zener diodes are used for the circuit described in Japanese Utility Model Laid-open No. Hei 5-006929, a variation in limiter voltage is directly caused, due to temperature dependence of a Zener voltage, so that the precision of the limiter voltage cannot be ensured.